Signal selecting system



April 13, 1937. W. A. MacDoNALD SIQNAL SELECTING SYSTEM 2 Sheets-snee?l1 Filed Dec. 20, 1953 l 20 fs) I i60 470 P15160 490 I I I J0 60 100 2001.00 |000 2000 `i000 |0000 Mom/LA T/o/v F/rEq/)EA/cy (cycles) ATTORNEY.S

April 13-1'9'31 w. A. MaoDoNALD SIGNAL sELEcTING SYSTEM Filed Dec. 20,1933` 2 Sheets-Sheet 2 INVENTOR ATTO R N EYS Patented Apr. 13, 1 937UNITED STATES 2,011,049 SIGNAL' Semeraro. SYSTEM william A. Mummia, nineNeck, N. Y., usignor to Hazeltine Corporation Application December 20,1933, Serial No. 703.218

13 Claims.

This invention relates to the reception and selection of modulatedcarrier signals, and has for its principal vobjects to facilitate theselection of such signals and to improve the fidelity of restances thesideband frequencies of one signalr channel either overlap those of anadjacent signal c 1.5 channel or else closely encroach upon them. Ineither case, it is diillcult, when tuning 'a radio broadcast receiver toa desired Asignal in one such channel, to eliminate interference due tosignals in the adjacent channels, particularly 20 when such interferingsignals are .received with a strength comparable to that of the desiredsignal.

Operation without interference in such cases requires that the selectingsystem shall tune to 25 a. suiiiciently narrow band of modulationfrequencies to prevent.` appreciable passage of the interfering signals.Narrowing the selected band in this manner tends to impair fidelity ofre- 30 ception of the signals-voice, music and the like,

the higher audio frequencies of modulation are suppressed. Accordingly,it is desired that the selected band width be allowed to remain narrowonly when interfering signals are present, but 35 in their absence, theselecting system Should be adjusted to freely admit and pass all of thereceived sidebands of the desired signal, thus preventing sidebandattenuation. According to this invention. there is provided a sidebandadmission system adapted to adjust the selectivity to admit the carrierfrequency and any portion of the two associated sidebands between aminimum band width and Substantially the entirev sideband width. Theadmission system comprises a selective coupling network for expanding orcontracting at will the selected or admitted frequency band width,independently of the tuning to the signal carrier channel.

A feature of the invention is the use of a selective coupling networkcomprising a plurality trol device is adapted to adjust the tunable cir-Y 55 'cuits either to be all resonant to the desired carsince thesideband frequencies corresponding to i (Ul. Z50-20) l rier frequency,or to be resonant `to diierent frequencies located somewhat above andbelow the said carrier frequency. When'the tunable circuits are alladjusted to be resonant to the carrier frequency, the network passes aminimum band width; but when the resonance frequencies of the tunablecircuits are shifted above and below the carrier frequency, the bandwidth .is increased until a maximum width is reached, which correspondsto the greatest separation of the resonances.

In tuningto the desired signal, the unitary control device is preferablyadjusted to produce the condition of minimum band width, or great- 'estselectivity; and upon completing the tuning operation, the band widthpassed by the selective coupling network is increased by manipulation ofthe control device to provide any desired degree of fidelity. c

In carrying out the invention, numerous combinations of tunableselective networks and vacuum tubes are possible. In the embodimentdescribed in detail hereinafter, thereis disclosed an arrangementincluding a pair of tunable circuits arranged eectively in parallel tosupply signal voltage to the input circuits of each of a pair ofrepeating vacuum tubes. v

In the drawings,

Fig. l` illustrates an adjustable carrier frequency sideband admissionsystem inaccordance with this invention;

Fig. 2 illustrates the selectivity characteristics of the system of Fig.1 under minimum and maxi- `mum expanded conditions;

Fig. 3 shows the selective characteristics of the severalresonantcircuits in the sideband admission system of Fig. 1;

Fig. 4 illustrates an embodiment of the inven-4 tion in asuperheterodyne radio receiver;-

Fig. 5 indicates the overall selectivity characteristics of the receiverof Fig. 4 as influenced by the side band admission system of thisinvention: and

Fig. 6 shows fidelity characteristics at the output oi' the receiver ofFig. 4 as influenced by the sideband admission system.

Fig, 1 shows a sideband admission y system adapted to be incorporatedinto a modulated carrier frequency portion of a signal translatingsystem. Thejselectings system is provided with D pacityCi and aninductance L1 in parallel, the

resonant circuit being permanently tuned to the desired carrierfrequency by the capacitively reactive and inductively reactive elementstherein. The coil `L1 is coupled to each of two adjustable resonantcircuits 2 and 3 by means of electromagnetic couplings M and M' to coilsLa and La situated respectively in circuits 2 and 3. Coll Le of circuit2 is tuned by a. xed condenser C: and a variable condenser C4, inparallel. Coil La of circuit' 3 is similarly tuned by a fixed condenserCa and a variable condenser Cs.

Resonant circuit 2 is connected between the control grid 6 and thecathode l of a pentode type vacuum tube V1, by means of connections Eand t, the latter including a blocking condenser iii. The cathode 'i isconnected to ground ii through a variable resistor l2. The tuned circuit3 is similarly connected between the control grid it and the cathode Itof a pentode type vacuum tube V2, by means of connections it and i6, thelatter including the blocking condenser l'i. The cathode i4 is connectedto ground through a variable resistor I 8. The anodes it and til,respectively, of tubes Vi and V2 are connected together and to theprimary circuit il of a carrier frequency coupling transformer T, whichconstitutes a resonant output, or signal response, circuit for tubes Viand V2. The primary circuit i comprises a coil L4 and a condenser Cc inparallel, the lower end of this parallel combination being connected toa, source of energizing voltage +B for the anodes of the' tubes, and theupper end being connected to the anodes. The secondary circuit t of thetuned transformer 'T comprises a secondary coil L5 coupled to coil L4,and a con-'- denser C7 connected across coil Is. The tuned circuits 6iand t are each made resonant at the carrier frequency to be passed; andthese circuits, and also circuit i, are designed to transmit theassociated sidebands of modulation as well as the carrier frequency. Thefrequency band-width transmitted by these circuits may be articiallybroadened by damping or other means, if desired. The output terminals O,O' of the selecting system are connected across condenser C1, and willordinarily beI connected to the input of a carrier frequency amplifiertube.

Although there are shown no cathode heating means or actual sources ofoperating potentials for the tube electrodes', these are all well knownto those skilled in the art, and therefore no further discussion isnecessary here.

There is provided a mechanical uni-control device, or unitary means,indicated by the dotted lines and knob U for simultaneously operatingthe variable condensers C4 and C5. These latter condensers are sorelated to each other by the mechanical, or manual, uni-control devicethat when one of them is in the condition o1' maximum capacity value,the other has a minimum capacity value. To place the resonant circuits 2and 3 in the operative condition, the mechanical control device U ismoved into one of its extreme positions, so that one of the variablecondensers C4 and C5 is at a maximum while the other is at a minimum. Inthis condition, the tuned circuits 2 and 3 are each adjusted to besharply resonant at the carrier frequency.

Upon moving the uni-control device out of the extreme position in whichthe above-mentioned adjustment was made, the capacity of one of thevariable condensers decreases while the capacity of the other variablecondenser increases. Hence, the resonance frequency of one of thesecircuits decreases somewhat while that ofthe other of these circuitsincreases. 'I'he result of this operation is to spread the individualresonance peaks oi resonance circuits 2 and 3 by an amount'dependentupon the adjustment of the device U. Since the anode circuits of tubesV1 and V2 are combined at circuit 4 of transformer T, these individualresonance peaks of selecting circuits 2 and 3 appear at the outputterminals O, O'. Therefore circuits 2 and 3 operate effectively inparallel between the input and output terminals, and the width of theband of frequencies at the output terminals may be the overall width ofthe individual resonance characteristics of circuits 2 and 3. Byadjustment of the continuously adjustable mechanical device U, the widthof the selected frequency band may be continuously varied from a minimumto a maximum, the minimum condition being that of greatest selectivityin which the two resonance peaks lie at the same -frequency, and themaximum condition being that of greatest fidelity, in which theresonance peaks, or transmitted bands, are displaced furtherest apart.

2 is an illustrative chart plotted in decibels gain against carrierfrequency in kilocycles, and shows characteristic curves of thetransmission efiiciency of the coupling network shown in Fig. l. Curve Ais the sharp resonance characterlstic obtained under the condition ofgreatest selectivity obtained when the mechanical control device U is atthe extreme position which causes the individual peaks of circuits`2 and3 to be both situated at the same carrier frequency. Under 'thecondition of characteristic A, only a portionof the usual full sldebandof frequencies are freely transmitted, this following from the fact thatthe circuits 2 and 3 individually pass only a portion of the full band.This carrier frequency to which the system is tuned to produce thecharacteristics 0i the chart is 175 kilocycles, this being a convenientcarrier frequency for use in the intermediate frequency amplifiers ofsuperheterodyne receivers. Curve B shows the double peak effect obtainedby movement of the control device U to the opposite extreme positionfrom that which gives characteristic A. The position of the controldevice which provides characteristic -A is called the position ofminimum band width, or minimum position, and the position which givescurve B is called the position of maximum band width, or maximumposition. It is observed that in moving to the maximum position, one of`the individual peaks has changed its position approximately 5 kilocyclesupward in the frequencyscale to about 180 kilocycles, while the otherpeak has changed its position downward by the same amount to aboutkilocycles.

1t is noted that greater gain at the carrier frequency is obtained inthe position of minimum band width than in the position of maximum bandwidth; this is due to the coincidence `of the two peaks composingcharacteristic A. The change in gain between the selective and expandedconditions may be largely compensated by a system of automatic volumecontrol, an appropriate form of which will be described hereinafter.

Fig. 3 is a chart of curves showing selectivity relationships betweenthe resonant circuits 2 and 3 and the transformer T, of Fig. l. In thisfigure, the ordinates represent percentage gain plotted on a logarithmicscale, and the abscissas` represent modulation sideband frequenciesplotted on a linear scale. The frequency marked 0" represents thecarrier frequency, while those frequencies on either side of the carrierrepresent resonance curve of one of the adjustable resonant l circuits 2or 3 of the selective coupling network, when tuned to the carrierfrequency. Curve E is the resonance characteristic of' circuits 2 and 3,in combinationwhen both are tuned to the car.. rier frequency under thecondition of maximum l selectivity. It is observed that curve E issimply a summation of two curves ofthe type of curve D, and correspondsto curve A of Fig. 2. The fact thatthe characteristics of circuits 2 and3 are additive is 'due to the fact that these circuits are arrangedeffectively in parallel at the input circuits of tubes V1 and V2,respectively.

Curve F is the resonance characteristic of the combination of circuits 2and 3 when one of these circuits is resonant 5` kilocycles above thecarrier frequency and the other circuit is resonant 5 kilocycles belowthe carrier frequency. Curve F is a summation oi' two curves such as Dwhen displaced 10 kilocycles apart, as mentioned, and corresponds tocurve B of Fig. 2.

Curve G is the overall resonance characteristic of the system includingthe double tuned transformer T and the adjustable circuits 2 and 3 whenadjusted to be resonant 10 kilocycles apart.

In other words, curve G is the resultant of curves Y C and F. Curves Cand F are combined with each other geometrically to produce theresultant curve G, since transformer T is arranged in tandem through thevacuum tubes V1 and V2 with respect to circuits 2 and 3. Hence, curve Gis l0 the product (instead of the sum) of curves C and F.

Although curves C and F have been multiplied directly to produce curveG, it should be under.-

' stood that the chart of Fig. 3 is simply intended l5 to illustraterelative characteristics. The actual amount of gain at the outputterminals O, O

j relative to the input terminals-I, I' will depend 'upon theamplification of tubes V1 and V2.

Hence, the illustration of the peaks of curve G i0 at approximately 100%gain and the illustration of the peaks of curves C. D and F atapproximately 10% gain is simply for the sake of convenience. v l

' It will be apparent that the gain with the ad- `5 justment representedby the curve F is very sub- .stantially less than that of 'curve E andmay be of the order of one-half the latter, as clearly shown in Fig. 3,where the percent gain is plotted on a logarithmic. scale. The reductionin gain, or re- `0 sponsiveness, which is incidental to the increasingof the band width, would result in unsatisfactory operation unlesscompensated for. 'I'he automatic volume control means, however, which ishereinafter described, provides the necessary `5 compensation andmaintains the output amplitude` of the system at a constant level.

Fig. 4 shows a complete superheterodyne radio receiver in theintermediate frequency amplifier of which the sideband admission systemin ac- 0 cordance with his invention is incorporated in the petitionshown at -X. `The receiver comprises a number of stages of vacuum tubeseffectively coupled in tandem by coupling systems. The details' of thereceiver, other than those associated 5 with the sideband admissionsystem, are of no special significance in so far as this invention is"concerned; hence, the receiver is described in general terms.In-examining Fig. 4, it should be understood that the terminals 60, 6|,62 and 63 at the dot-dash line PP beneath the first audio amplifier 41are intended to be connected to, or

identical with, the same-numbered terminals be-` neath the detector tube39. The circuit arrangement has been broken at PP simply for conveniencein illustrating the circuit arrangement on a single sheet.

'Ihe receiver comprises a signal pick-up system including an antenna 2|and ground Il. The

ing an oscillator tube 30. 'I'he output of the.

oscillator system appears in a coil 3i which is included in thegrid-cathode circuit of modulator 29 for the purpose of producing themodulation in a manner wellunderstood in the art. Since the assemblingof such oscillator and modulator circuits is now well known, no detailsof these circuits need be given here, since they constitute no part ofthis invention. r

In accordance with the well known phenomenon of modulation, thereappears in the output circuit of the modulator, that is, the circuitincluding the anode 32 and the cathode 28, the products of modulation.These products of modulation include the band of frequencies which arethe difference between the band of frequencies of the received signalchannel and the local oscillator frequency. 'I'his band of dierencefrequencies is known as the intermediate frequency band, and comprisesthe intermediate carrier frequency and the upper and lower sidebands ofmodulation. Since ordinary radio broadcast carrier signals are modulatedby audio frequencies up. to about 6 kilocycles, each of these sidebandsis about 6 kilocycles in width, andthe upper and lower sidebandstogether occupy a channel band width of about l2 kilocycles. Forexample. if the frequency of the local oscillator 30 be adjusted toproduceanv intermediate carrier frequency of 175 kilocycles, as shown inthe chart of Fig. 2, the frequencies of the intermediate frequencychannel occupy the band of about |69 to IBI kilocycles.

Accordingly, the selective system of an intermediate frequency amplifier`is proportioned to select and pass approximately the full band width ofthe order of 12 kilocycles. Such a full selected band width permits ahigh degree of fidelity of reception, but-does not result 'in as high adegree of selectivity as would be provided by a more sharply tunedselecting system which would not permit the full sideband width to bepassed. In accordance with this invention, it is possible to adjust thebandof frequencies permitted to be passed and amplified by theintermediate frequency amplifier to any width between a maximum ofapproximately the full sideband width of l2 or more kilocycles to aminimum of a, much lesserwidth. v

The input terminals I and I' of the adjustable sideband admission systemX of this invention are effectively connected to the anode 32 and thecathode 28, respectively. Terminal I is directly connected to anode 32,and terminal I is con- 5 nected through a -condenser 33 to ground, and

thence to the cathode 28 through appropriate bypass condensers. Byreason of the opposite variations of the capacities of variablecondensers C4 and Cs, when operated by the mechanical conm trol device,the band of frequencies admitted 'by the cooperative action of resonantcircuits 2 and 3 may be varied from a minimum of about 6 l kilocycles toa maximum of over 12 kilocycles. Hence, the actual band width of thesignals at 15 the output terminals O, O of the admission system variesaccording to the adjustment of condensers C4 and C5.

The output terminals O and O are effectively connected to the controlgrid t5 and the cathode t5, respectively, of an intermediate frequencyamplifier tube 31. The output of amplifier t1 is coupled through anintermediate frequency coupling system 38 to a diode type of detector35. Although this diode type detector is shown with three electrodes,the cathode 55 and the plate il are connected together, as shown, toconstitute a single electrode which is the cathode.

There is connected between the cathode dt (the ground connection) andthe low potential end of the secondary circuit of coupling system t8, apair of series-connected resistances d2 and t3 acrosswhich are developedthe rectied components of the detector output. The audiofrequencycomponent is taken from that portion of resistor i2 between ground andthe tap dfi, and impressed through a condenser 135 upon the input gridi6 of an audio-frequency, ormodulation frequency, amplifier tube 51. Asecond audio-frequency amplifier tube dd is coupled to the output oftubed1 by resistance coupling, as shown. The last audio-frequencyampiier stage comprises audio-frequency amplifier tubes 49 and 50arranged in the Well-known push-pull relation. vFor the purpose ofconverting the audio-frequency currents into sound Waves, there isconnected at the output of the push-pull arnpliier stage a doubleloudspeaker arrangement 5i.

In addition to the above-mentioned circuits, 50 the receiver is providedwith a system of automatic volume control of the type described inUnited States Patent 1,879,863 to Harold A. Wheeler. 'I'his system,comprises a connectionv 52 from the junction 53 between resistors 42and 55 43, to the control grids 23, 21, 6 and I3 of tubes 25, 29 and V1and V2, respectively. In this automatic volume control system, there isdeveloped across resistor 42 a uni-directional voltage which varieswith4 the average carrier voltage impressed on the diode detector 39.Due to the detector action, the current in resistor 42 flows from groundtoward the point 53. Hence, the point 53, and therefore the controlgrids 23, 21, 6 and I3 become more negative when the signal strength atthe dector increases, and vice versa. This causes the amplication oftubes 25, 29 and V1 and V2 to decrease when the signal strength at thedetector increases, and to increase when the signal strength at thedetector decreases. Consequently, the automatic volume control systemfunctions to compensate for changes in the received signal intensity andalso for changes in the transmission eillciency of the sidebandadmission system when the latter is adjusted between the maximum andminimum positions; and

the output voltages of the detector 39 are maintained substantiallyconstant.

For the purpose of furnishing operating voltages for the electrodes ofthe vacuum tubes, there is provided a power supply system indicatedgenerally at 54. This may be a conventional power system adapted tooperate from ordinary alternating current mains, such as the usual 60-cycle house current. Since suitable power systems for supplying properelectrode voltages and for heating cathodes of tubes are well known inthe art, no detailed discussion is necessary.

Throughout the receiver there are employed condensers and resistorswhere their use improves the operation of the receiver.

Fig. 5 is an illustrative chart oflpercentage gain plotted on alogarithmic scale against modulation sideband frequencies plotted on alinear scale. This chart shows the overall response of the receiver ofFig. 'i under the conditions of minimum band width and of maximum orexpanded band width. In this chart, the frequency marked 0 representsthe carrier frequency, while those on either side represent themodulation frequencies of the sidebands in terms of the correspondingaudio frequencies. Curve H indicates the selectivity characteristic ofthe receiver under the condition of greatest selectivity,

that is, when the adjustable resonant circuits 2 and 3 are both tuned tothe intermediate carrier frequency.l Curve J shows the selectivitycharacteristic of the receiver when the sideband admission system isexpanded to admit the maximum band width. For the purpose of easycomparison, the peaks of both curves have been brought to approximatelythe same level, which would be the effect of the automatic volumecontrol operation described above. Curves H and J illustrate the rapidexpansion of the band near the peak of the characteristic in comparisonwith the expansion near the base where there is high attenuation at alladjustments. It s observed that at 50% of unity, or peak gain, the totalWidth of the two sidebands which is admitted is 5.5 kilocycles, for thehighly selective adjustment, while for the maximum expanded adjustment,it is about 13 kilocycles. This represents a band width expansion of136%. At .1% of the peak gain, however, the band width is expanded from22.5 kilocycles to 35.5 kilocycles in passing from the most selective tothe most expanded adjustment. This represents an expansion of only 58%.

Fig. 6 is a chart Whichshows the overall fidelity characteristic asmeasured at the output of the audio amplifier under the most selectiveand most expanded conditions. In this chart, the

Cal

percentage of audio-frequency output at 400 cycles is plotted againstthe modulation frequencies, or audio frequencies, of the 'entireaudiofrequency range. Curve K illustrates the fidelity characteristicunder the highly selective adjustment of the admission system, and curveL shows the fidelity characteristic when the selector is in its expandedadjustment. From these curves,

it is observed that the upper limit of the audiodit portions relative tothe other portion, whereby the resultant width oi.' the selected bandmay be 4 l is tuned to select and pass said carrier `frequency andsubstantially the entire range of the sidebands of modulation, and atuned output circuit, a pair of adjustable selecting circuits arrangedeffectively in parallel between said input and 2o output circuits, and'means for adjusting one of said selecting circuits relative to theother so that their resonant periods may be made to coincide or tobecome displaced relatively to each other, whereby the resultant bandwidth at said 25 output circuit may be expwded from a minimum width.

4. A modulated carrier frequency band selecting system comprising a pairof adjustable band selecting paths arranged eectively in parallel be- 30tween a pair of input terminals and a pair of output terminals, each ofsaid. paths comprising a, resonant circuit having an adjustable elementconnected between a control grid and cathode of a vacuum tube, each ofsaid tubes having an anode, .'55 which anodes are connected together inaiding phase and coupled to said output terminals, said adjustableelements being simultaneously operated by a single control device whichshifts the resonance frequencies of said circuits in opposite relativedirections in the frequency scale, whereby the band width of frequenciestransmitted to said output terminals may be varied.

5. A modulated carrier frequency signal se- -lecting system comprisinga, resonant input circuit and a resonant output circuit each tuned toselect vthe carrier frequency and associated sidebands, a pair ofparallel adjustably tuned circuits coupled to said input circuit, eachof said lust able circuits being coupled tothe input' circuit of avacuum tube, the output circuits of said tubes being connected togetherand to said ronant output circuit, and a manual control device forsimultaneously varying the resonancefrequency of said adjustablecircuits oppositely relatively to each other, whereby the frequency bandwidth at said output circuit may be regulated.

6. In a signal selecting system, a first and a second selective circuit,said circuits being arranged in parallel with common input and commonoutput connections and each circuit including inductively reactive andcapacitively reactive elements, means for adjusting the resonantirequencies ofl the elements in each of said circuits to be the same,and unitary control means whereby the resonant frequency of said firstcircuit may be decreased and the resonant frequency of said secondcircuit may be increased.

7. In a radio receiver, means for preventing sideband attenuation bybroadening the frequency admission band comprising a pair of adiustablyselective .resonant circuits connected in parallel and a signalresponsive circuit connected to said parallel circuits, vacuum tuberepeater means connected between said selective and responsive 75circuits, and unitary means for simultaneously increasing the resonantfrequency of one of said selective circuits and decreasing the resonantfrequency of the4 other of said selective circuits.

8. In a carrier current amplifier, a plurality of vacuum tubes anda'plurality of coupling systems for coupling said tubes in tandem, afirst of said coupling systems being resonant to the frequency of thecarrier current to be received, a second of said systems comprising aplurality of circuits connectedin parallel and resonant to frequenciessomewhat greater and les than said carrier current, and continuouslyadjustable unitary control means for alteringthe resonant frequencieslof said parallel circuits relatively to each other.

9. In a carrier current amplifier, a yplurality of vacuum tubes and aplurality of coupling systems for coupling said tubes in tandem, thegain of the ilrst of said coupling systems being maximum at thefrequency of the carrier current, the second of said coupling systemsincluding a plurality of circuits connected in parallel and tunable todierent resonant frequencies whereby the gain of said second couplingsystem 'is maximum for frequencies higher and lower than that' of saidcarrier current, and continuously adjustable unitary control" means forsimultaneously tuning said parallel circuits individually in oppositesenses to change the frequencies at which the gain in said secondcoupling system becomes maximum.

l0. In a carrier current ampliiier, a plurality of vacuum tubes and aplurality of coupling systems for coupling said tubes in tandem, meansfor resonating'the lfirst of said systems to the frequency of a carriercurrent, and a second of said coupling? systems including a plurality ofcircuits connected in piel and means for resonatingjsaid circuits to thesame resonant frequency. the second of said means comprising 4acontinuously adjustable control member for said parallel circuits forsimultaneously changing the resonant frequencies of said circuitsindividually.

and below said carrier frequency, where the selected band wid is amaiximum, whereby the ratio of the said mammum to the said minimum bandwidth at 50% of the peak gain is substan-` tially greater than at .1% ofthe peak gain.

l2. In a, carrier current amplifier, a plurality of vacuum tubes and aplurality of coupling systems for coupling said tubes in tandem, meansfor permanently resonating at least one of'said coupling systems to thefrequency of the carrier current, another of said coupling systemscomprising a plurality of resonant circuits connected in parallel, andmeans including a continuously adjustable unitary control member forsimultaneously altering similar reactive elements of said resonantcircuits individually in opposite senses to change the resonantfrequencies of said lastnamed resonant circuits individually'in oppositesenses between the limits-of the carrier current frequency andkilocycles removed therefrom.

13. In a signaling system\adapted to amplify a carrier frequency signaland the associated sidebands of modulation, a ilrst and a seconddetector, a vacuum tube ampliiiei' and a plurality ot coupling systemsarranged in tandem between said detectors, means for causing saidcoupling systems to be resonant at said carrier frequency so that thetransmission eiliciency is greater at the carrier than at the sidebandfrequencies, one of said coupling systems including a plurality oftunable coupling circuits connected in parallel l0 and having adjustablereactance elements oper- A. MACDOINALD. 10

